The development of a photodiode array (PDA) capable of acting as a photodetector over a wide spectral range as dispersed by a monochromator and sequentially reading out the intensity of separate wavelengths within this range by digital means has revolutionized the operation of scanning spectrophotometers. The multiple, linearly disposed elements of the PDA take the place of the exit slit and eliminate the need for mechanical scanning. However, because of the microscopic dimensions of these elements, extreme accuracy in location of the spectrum on the array has been required in order to maintain the correct wavelength on each element to be correctly reported on the readout display.
If the position of the spectrum is fixed, the array wavelength calibration is a matter of initial adjustment and requires proper design to eliminate secular changes due to temperature expansion, creep of structural materials, etc. If more than one value of dispersion is to be provided for higher resolution over shorter wavelength ranges, then a means of reestablishing wavelength calibration becomes essential.
It is an object of this invention to provide automatic microprocessor controlled means to establish initial wavelength calibration for a multiplicity of wavelength ranges and resolutions.
It is a further objective to have this initialization system incorporate checks of internal consistency of the instrument including corrections for temperature drifts and other changes affecting wavelength determination.
It is a further objective to make this wavelength establishing procedure available at any time to the operator through a single command.
It is yet a further objective to make this procedure programmable so as to be automatically called up whenever required including, for example, at turn-on and at appropriate times during operation.
Another objective of this system is to use an accurate emission line of known wavelength as the wavelength reference rather than any mechanical index.
Another objective is to provide in software, corrections for nonlinearity of the wavelength scale as projected on the linear array.
It is also an objective to make adjustments by the operator unnecessary.
Yet another objective is that any automatic adjustment be made through software rather than by mechanical motions.
It is also part of the object that the procedure be selfindexing and self-optimizing.